This is a souterrain, a type of stone-lined subterranean structure found across early medieval Ireland, typically associated with ringforts and used for storage, refuge, or both. What makes this example particularly notable is not just its length but its state of preservation and the deliberate complexity of its internal layout, which would have made it very difficult to navigate quickly in the dark.

The structure sits in the north-western quadrant of a rath, the Irish term for a roughly circular earthen enclosure that served as a farmstead in the early medieval period. The souterrain's first chamber is the longest at 15 metres, running roughly north-north-west to south-south-east before curving eastward into the first of two drop-hole creeps. A creep is a deliberately low and narrow connecting passage, sometimes with a step down, designed to force anyone moving through it to crouch or crawl, slowing intruders considerably. This first creep, just 75 centimetres high, leads into a second and third chamber. The third chamber then feeds, via another tight creep only 53 centimetres in height, into a fourth chamber running north-north-west to south-south-east. The overall effect is a passage that doubles back and narrows repeatedly, a design that is functional rather than incidental. The structure was recorded by McCaffrey in 1952 and subsequently by Korff and O'Connell in 1985, the latter survey forming part of a broader Galway archaeological study carried out through University College Galway.